The present invention relates to an electrode for welding comprising a guide hole of a circular cross-section composed of a small-diameter hole and a large-diameter hole and a guide pin composed of a small-diameter portion and a large-diameter portion, the small-diameter portion and larger-diameter portion of the guide pin being fitted into the small-diameter hole and large-diameter hole of the guide hole, respectively, so that when the guide pin is displaced relative to the guide hole compressed air is allowed to jet out of a gap between the small-diameter hole and the small-diameter portion.
The prior art most closely related with the present invention is described in Japanese Utility Model Publication 54-9849 and Japanese Utility Model Publication 62-32714. The former is as shown in FIG. 11, and the latter in FIG. 12. Referring first to FIG. 11, a guide hole 2 in an electrode 1 is composed of a small-diameter hole 3 and a large-diameter hole 4, a conical seat 5 connecting the cylindrical walls of the holes 3 and 4. Similarly, a guide pin 6 is composed of a small-diameter portion 7 and a large-diameter portion 8, a conical portion 9 connecting the portions 7 and 8. A coil spring 10 is inserted in the guide hole 2 to push the guide pin 6 upward. An air port 11 for introducing compressed air is formed in the electrode 1. As shown, the conical portion 9 is in tight contact with the conical seat 5 under the action of the coil spring 10. A gap 12 is provided between the small-diameter hole 3 and small-diameter portion 7. A steel plate part 13 is positioned on the electrode 1 with the guide pin 6 passing through an opening formed in the part 13 and a tapered end 15 within a threaded hole of a projection nut 14 so as to be ready for the forward movement of a movable electrode (not shown) situated above the electrode 1. The electrode 1, made of copper alloy, is composed of a main body 16 and a cap 17 integrated through threaded parts 18, the outer shape and the guide hole 2 being both circular in cross-section. As the movable electrode advances to push the guide pin 6 down, the conical portion 9 is separated from the conical seat 5 to thereby allow compressed air from the air port 11 to pass through an annular gap thus formed over the conical portion and to jet out of the gap 12. As the movable electrode further advances, the projection nut 14 is pressed against the steel plate part 13, and when a welding current is turned on, the nut 14 and the steel plate part 13 are fusion-welded together. Spatter, which is produced during the process of fusion welding, is blown away by the air which jets out of the gap 12 and is prevented from entering the gap 12. The current of air also affords cooling to the electrode.
Referring to the prior art of FIG. 12, members having the same functions as in FIG. 11 are identified with same reference numerals as in FIG. 11 and their detailed description is omitted. An annular end surface, or a shoulder, 19 radially extending in a plane perpendicular to a longitudinal axis of the guide pin 6 is formed in the boundary of the large-diameter portion 8 and small-diameter portion 7, while an inner end surface, or an annular seat, 20 of the large-diameter hole 4 is formed in the boundary of the large-diameter hole 4 and small-diameter hole 3, with an O-ring for airtight sealing fitted into the large-diameter portion 8. The operation in the FIG. 12 arrangement is similar to that described in relation to FIG. 11 with the exception that compressed air does not jet out of the gap 12 but is fed simply to push up the guide pin 6, while the air tightness is kept by the O-ring. Hence, no countermeasure against nor cooling action is contemplated in this case.
The prior art had the following problems. In the FIG. 11 arrangement, when the guide pin 6 is pushed down to separate the conical portion and the conical seat, the guide pin 6 comes to float, and therefore, the guide pin 6 can itself become eccentric in the guide hole 2 until the projection nut 14 is pressed against the steel plate part 13, causing the nut 14 to be welded to an improper position on the steel plate part 13. Moreover, unless the cone angle of the conical seat 5 and conical portion 9 is finished to an extremely high precision, the conical portion cannot come in contact with the conical seat in an airtight fashion, leading to plant air leaks which is very uneconomical. The problem inherent in the FIG. 12 arrangement is in that no scattering of spatter nor air cooling is available. That is, no consideration is given to spatter treatment and air cooling.
Although not specified in the utility model publication, in this type of guide pin, the entire surface is coated with ceramic for insulation and wear resistance. This is as shown in FIG. 13; for example, ceramic is sprayed to the surface of the guide pin 6 made of metal such as steel to form a coating layer 19. In such guide pin as described the coating layer 19 with a very rigid and rough surface can be extremely worn by the small-diameter hole 3 and conical seat 5, which results in that the relative position of the guide pin 6 itself with respect to the cap 17 in the diametrical direction, that is, the centering cannot be achieved sufficiently. This problem is particularly significant when the inner surface of the small-diameter hole 3 is worn. Moreover, when the conical part 15 is worn by the corner edges of the projection nut 14 so that the coating layer 19 is worn out to the extent that the nut comes into direct contact with the metal portion, the intrinsic insulating function is sacrificed.